(1) Field of the Invention
The invention relates to detection of acoustic signals from beneath a water surface and is directed more particularly to detection of signals from an underwater source, wherein the acoustic signals are present in random noise, and which heretofore have gone undetected.
(2) Description of the Prior Art
Conventional acoustic signal processing extracts information from sonar-received signals, using time or frequency domain quantities obtained using spectral analysis. Q. T. Zhang has described an entropy-based receiver for detecting correlated signals present in white noise, both having unknown statistics, as discussed in "An Entropy-based Receiver for the Detection of Random Signals and Its Application to Radar Signal Processing", by Q. T. Zhang, Signal Processing, Vol. 18, pp. 387-396, Elsvier Science Publishers (1989). The basic idea behind the entropy-based approach is that under constant energy constraint, the entropy of a received process having a finite number of sample points is maximum when the process is white (i.e., random) and decreases as it becomes correlated. Results based on simulated data and actual radar data are found in the aforementioned Zhang publication and in "Elements of Information Theory", by T. M. Cover and A. T. Joy, p. 239 (1991). M. S. Hughes has described a novel symbol-oriented information theoretical approach to signal analysis based on the application of Shannon entropy. Hughes shows that Shannon entropy processing can be a preferred alternative to conventional signal analysis by making an experimental comparison of scanned ultrasonic images. Hughes has applied the entropy concept to the analysis of signals observed in ultrasonic experiments and successfully demonstrated the classification of various defects in plexiglass. His work is described in "Analysis of Digitized Waveforms Using Shannon Entropy", by M. S. Hughes, JASA, Vol. 93, No. 2, pp. 892-906 (1993). N. J. I. Mars has exploited mutual information in time delay estimation for electroencephalographic signals, as discussed in "Time Delay Estimator for EEG Analysis Based on Information Theory Coherence and Time Delay Estimation", by N. J. I. Mars, IEEE Press, pp. 410-412 (1993).
All of the research efforts mentioned above are related to sonar detection, classification and range estimation. However, the detection of acoustic signals emanating from an underwater source, and buried in random sea noise, is beyond the scope of the prior art teachings.
In a military environment at sea, there is a need for a method to detect and analyze such acoustic signals to make known the presence of an underwater weapon or weapon-capable platform.